Many applications involve the interaction of liquids with solid surfaces. Often, it is desirable to control or influence the manner of the interaction, particularly the degree of wetting of the surface, so as to achieve a specific result. As an example, surfactants are sometimes added to liquids used in cleaning processes to achieve increased surface wetting. Conversely, liquid repellent coatings are sometimes added to products to reduce surface wetting and accelerate drying of the surface.
The principles and properties affecting surface wetting have been studied for decades to understand physical/chemical interactions that effect the nature of the surface. There has been and continues to be a particular interest in surfaces that are resistant to wetting by liquids. Such surfaces are referred to as hydrophobic where the liquid is water, and lyophobic relative to other liquids. If the surface resists wetting where a small droplet of water or other liquid exhibits a very high stationary contact angle with the surface (greater than about 120 degrees), if the surface exhibits a markedly reduced propensity to retain liquid droplets, or if a liquid-gas-solid interface exists at the surface when completely submerged in liquid, the surface is generally referred to as an ultra hydrophobic or ultra lyophobic surface.
Likewise, where a small droplet of water exhibits a stationary contact angle with the surface that is greater than about 150 degrees, the surface is generally referred to as super hydrophobic.
Ultra hydrophobic and super hydrophobic surfaces are of interest in commercial and industrial applications. In nearly any process where a liquid must be dried from a surface, it is most efficient if the surface sheds the liquid without heating or extensive drying time.
Friction between the liquid and the surface is dramatically lower for an ultra hydrophobic or super hydrophobic surface as opposed to a conventional surface. As a result, ultra and super hydrophobic surfaces are extremely desirable for reducing surface friction and increasing flow in a myriad of hydraulic and hydrodynamic applications on a macro scale, and especially in microfluidic applications.
It is now understood that surface roughness has an effect on the degree of surface wetting. It has been generally observed that, under some circumstances, roughness can cause liquid to adhere more strongly to the surface than to a corresponding smooth surface. Under other circumstances, however, roughness may cause the liquid to adhere less strongly to the rough surface than the smooth surface. In some circumstances, the surface may be ultra or super hydrophobic.
The roughness, it is believed, helps to reduce the adhesion of the surface for polar liquids such as water. Roughness also appears to lead to reduced adhesion of solid deposits such as dirt particles on the surface. Under appropriate conditions, surfaces that are roughened and hydrophobic, dirt particles are flushed from the surface by moving water. This effect is referred to as the self-cleaning effect or lotus effect.
However, it has been found that most of the super or ultra hydrophobic surfaces are often formed with a delicate polymer or chemical coating that is deposited on the substrate surface. These coatings tend to be easily physically damaged so as to be not as effective as desired.
Therefore, a need exists for super or ultra hydrophobic coating compositions that are easily prepared and can withstand common usage in a given application.